Both "빨래하다" and "세탁하다" have similar meaning, which is to clean some clothes. But "세탁하다" means to clean clothes with machine. "빨래하다" means to clean clothes with machine or hands. And "세탁하다" is used, when you clean some others. (Like money-layndering, you make some illegal things to legal things) "빨래하다" is used to clean only clothes.